Generally, a disposable absorbent article, such as a diaper, has several adhesives because various parts of the article require different functionality: core adhesives add strength to the diaper pad when it is wet; construction adhesives bind the waterproof backsheet to the nonwoven absorbent pads; and elastic adhesives bind legs, waist and lateral panel sheets. While the aforementioned adhesives perform different functionality, all of them typically require significant amounts of tackifier and/or diluents to be useful as adhesives.
Olefin-based adhesives have typically been used as core and construction adhesives. While widely used, olefin-based adhesives require large amounts of tackifiers and plasticizers to be effective absorbent article adhesives. For example, sprayable ethylene and/or maleic acid modified ethylene α-olefin polymer based adhesives typically require greater than 30 wt % of tackifiers and/or diluents. Use of high levels of oil in adhesives can lead to oil migration out of substrates at elevated temperature.
Amorphous atactic poly-α-olefins (APAO) adhesives are widely used as construction adhesives for nonwovens. These adhesives are typically chosen as construction and/or core adhesive, and not selected as elastic adhesives.
A blend of atactic and isotactic polyethylene polymers (IPP) and a blend of atactic and sydiotactic polypropylene (SPP) have been used to balance the cohesive and adhesive strength of the adhesive. Even with the balanced properties of cohesive and adhesive strength, the above adhesives do not have the required performance characteristics of an elastic adhesive.
While recent polymer technology has introduced block olefin polymers which allows for better creep resistance, large quantities of low molecular weight plasticizers are required in the adhesives, and this negates the creep performance at elevated temperatures. The amount of the block olefin polymer in the adhesive is less than about 50 wt %, preferably less than 30 wt %, and most preferably less than about 20 wt %, based on the total weight of the adhesive. Adhesives made with polymers that have blocks of regular isotactic structure, interdispersed by segments of atactic structure, still require at least 30, and up to 70 wt % of a tackifier, based on the total weight of the adhesives.
Because of the insufficient elastic performance, many commercially available elastic adhesives are styrene block copolymers based. Adhesives made from such block copolymers are readily available and are described in the art. The hard block of styrenes anchor onto the substrate and hold its shape, allowing for elasticity. However, styrene-based adhesives also require large quantities of tackifier and/or diluents to balance the application temperature and viscosity.
While low crystalline (less than 10%) content adhesives based on butene poly-α-olefin are known, such adhesives are not suitable for elastic substrates due to its stiffness.
There continues to be a need in the art for cost-effective adhesives with desirable cohesion and creep resistance with low tackifiers and plasticizer content. The current invention addresses this need.